Three-phase high-voltage distribution and transmission lines consist of three energized conductors and a fourth conductor that is the neutral or “ground” conductor. Each of the three energized conductors carries an electrical voltage that varies in magnitude at the same frequency but the phase of the voltage carried by each conductor is displaced by a phase angle of 120 degrees. These three conductors are generally identified as the A, B, C or 1, 2, 3 conductors, or equivalent, depending on the utility, to tell them apart. In the simplest arrangement, the first phase, which may be the A or 1 conductor, serves as the reference phase and is arbitrarily designated to be 0 degrees, making the next phase 120 degrees displaced from the first and the last phase 240 degrees displaced from the first.
When two sets of high voltage distribution and transmission lines are to be connected, the phases of each line must match: an A phase conductor of one set is to be connected to an A phase conductor of the other set, and so on for each of the conductors. In total, there are six possible ways to attach any two sets of three conductors. Each of these six different connections will result in a different outcome for the device being powered—an outcome that is significant. Incorrectly-wired three-phase transformer banks, consisting of three individual transformers, for example, can produce phase angles between 0 and 360 degrees in 30 degree steps. Accordingly, correct phase identification prior to connecting conductors is an important practical and safety concern to those who maintain high voltage distribution and transmission systems.
Unfortunately, the phase identity of an individual conductor may be difficult to trace in overhead distribution and transmission systems, and very difficult to trace in underground electrical systems, which may extend for many miles. Moreover, unauthorized digging or trenching up of an underground electrical system, unfortunately a common occurrence, may sever conductors and result in loss of phase identification. Natural disasters such as accidents, hurricanes, tornadoes, forest fires, high winds, snow, ice, earthquakes, floods, etc. may also result in loss of phase identification in above-ground and underground transmission systems. Construction and restoration of these systems and verification of system records require accurate phase identification, conductor tagging, and mapping of the electrical transmission lines.
Measuring instantaneous voltages of two conductors is part of the process of determining their time-varying voltages. When conductors are far apart, their separation distance introduces errors in comparing any two measurements. Eliminating, correcting or avoiding those error is vital to correctly identifying the phases of two separated conductors. Safely making these connections with minimum use of communications bandwidth and a minimum amount of data transfer is advantageous simply because the communications may be taking place under difficult circumstances, or over great distances, and the transmission errors that will often occur under these circumstances can have serious consequences.
Measuring the phase difference indirectly between the voltages on electrical conductors per se is known. One system, disclosed in U.S. Pat. No. 6,642,700 issued to Slade et al and assigned to Avistar Inc., identifies phase angles of electrical conductors in remote locations by measuring the time delay between an external clock source and a zero crossing of the waveform. A time tag is associated with that time delay and transmitted over a full-duplex communications link between a field unit and a reference unit. At the reference unit, the phase angle is calculated from the time tag and displayed. The Avistar system uses the global positioning satellite (GPS) system as its external clock for determining the time delay. In order for this system to operate in real time, it requires either a half-duplex or full duplex, full-time, communications link of relatively high speed to transfer the time tag and the voltage information.
Another phase angle measurement system is described in U.S. Pat. Nos. 6,734,658 and 7,109,699, issued to the present inventor. In this system, a signal that has been corrected for capacitive charging currents is obtained by a master probe measuring the voltage carried by a conductor in the field. The phase of a signal from the master probe is compared to the phase of another signal from a supplemental probe that has measured a reference voltage and is then transmitted wirelessly and in full duplex from the supplemental probe. The phase difference is then displayed by the master probe. This system compensates for the phase shift introduced when a signal is sent from one probe to the other. The transmitted voltage signal is encoded onto a carrier wave by modulating that wave with the voltage information. This system also requires the use of a full-time half-duplex or a full duplex communications channel of relatively high speed.
A third system is described in U.S. Pat. Nos. 6,734,658, 7,808,228, 8,283,910, and 8,283,911, issued to the present inventor. In this system, the phase of a voltage carried by a reference conductor is measured by a reference probe and compared to the phase of a precision 60 Hz waveform generated from a GPS receiver signal. The phase difference between these two waves, in the form of a nine-bit data signal is transmitted over a distance, perhaps miles, to a receiver that decodes the data signal and uses another precision 60 Hz waveform generated by another GPS receiver to re-create a surrogate reference wave identical to the original reference voltage. This surrogate wave is forwarded to a (nearby) meter probe that is measuring the voltage on a field conductor. The meter probe can then compare the two waves to determine the phase angle difference between them. This system represents an improvement over the previous two systems relating to the communications requirements because it only requires a low-speed, simplex data channel.
These three prior art systems use different ways to obtain and compare signals that represent the phases of the voltages carried by the reference and field conductors and have different communications requirements. The Avistar system compares time tags of the field and reference voltage signals, wherein the time tag of each is the difference between a GPS time and the zero crossing time of the alternating voltage, to determine the phase difference between the two time-varying voltages.
The first Bierer system compares the phase of the reference conductor voltage to that of the field conductor voltage directly but compensates for the phase shift of the transmitted reference voltage resulting from the transmission distance to the master probe measuring the field conductor voltage.
The second Bierer system determines the phase angle between a reference conductor voltage and a precision 60 Hz waveform generated from a GPS signal. This phase angle is transmitted to a distant receiver where a surrogate of the original reference conductor voltage is being re-created. The phase difference between the surrogate waveform voltage carried by a field conductor are then compared.
There remains a need for high voltage phasing voltmeter that is accurate, easy to read, and useable when the high voltage distribution or transmission lines are many miles apart or when the electric power grid is not operating at its full nominal frequency. Therefore, a system that meets this need can operate notwithstanding lower quality data communications over greater distances, such as cell phone data links, and still accurately and reliably enable the utility worker to determine the phases of electrical conductors.